With a broadband mutilmode electro optically Q switched Cr:LiSAF laser pulse passing through several dispersion prisms, its frequency band is dispersed. Then by focuing it into CS 2 cell the backward stimulated Brillouin scattering is produced. Experimental results show that in comparison with case of no-dispersing prism frequency band dispersing method can efficiently reduce the broadband SBS energy threshold, raise pulsewidth compression ratio by more than 5 times, and improve beam quality of pump light.